A phrase for use to highlight an ensuing awkward or unbelievable event. Mentioning that it is 'actually happening' helps to address either the awkwardness or awesomeness in the situation to everyone present. Therefore, can be used as a positive reinforcement to encourage the behaviour or as a negative reinforcement to discourage the offender.

Use in positive situation: Three hot women approach man and his friends in a bar and ask if they can join them.
Lucky Man: "This is actually happening."

Use in negative situation: A friend among a group starts to complain about other members of the group in an irrational and mood-ruining manner.
Any other group member: "This is actually happening."